Process for assembling substrates with low-temperature heat treatments

ABSTRACT

The invention relates to a process for producing a bond between a first and a second substrate ( 2, 4 ), comprising:
         a) a step of preparing surfaces ( 6, 8 ) to be assembled,   b) an assembly of these two surfaces, by direct molecular bonding,   c) a heat treatment step involving at least maintaining the temperature within the range of 50° C. to 100° C. for at least one hour.

TECHNICAL FIELD AND PRIOR ART

The invention relates to techniques for assembling substrates.

In general, a bond between two substrates or surfaces can be obtainedafter a preparation of the surfaces giving them a hydrophilic orhydrophobic character.

The use of heat treatments to reinforce the direct bonding can cause,for a certain number of bonded structures, the appearance of defects atthe bonding interface. These defects are due to the degassing ofby-products of the molecular bonding reaction: for example, water,hydrogen or hydrocarbon molecules.

For a certain number of bonded structures, it is known that thesedefects can be resorbed by heat treatments performed at very hightemperatures. These temperatures are, for example, between 900° C. and1300° C. and are based on the preparation of surfaces before bonding.Unfortunately, for other bonded structures, this solution cannot beused.

The limitation of the surface oxide thickness or the presence of variousmaterials facilitates the appearance of defects at the bondinginterface.

In the case of thin films (with a thickness below around ten μm orseveral dozen nm), heat treatments, at temperatures below 1000° C., forexample between 600° C. and 800° C., cause the formation of bondingdefects in the form of blisters or zones without adherent film. Thesedefects cannot be suppressed by higher-temperature heat treatments. Forexample, the bursting of bubbles is promoted by the fineness of thelayers. These defects make the structures produced unusable. Currently,this phenomenon limits the production of oxide film structures, embeddedat the bonding interface, that are fine (thickness below 50 nm) orultra-fine or even Si layers directly bonded to Si plates.

Similarly, for heterostructures (for example P-doped Si bonded toN-doped Si), heat treatments cause the formation of bonding defectsunder certain conditions. A high-temperature heat treatment (1000° C.)would cause interdiffusion of the doping agents.

For certain heterostructures, if the damage is excessive in heattreatments within the temperature range below 800° C., this damage canno longer be repaired by a treatment between 1100° C. and 1300° C.

When the heat treatments at higher temperatures cannot be used(incompatibility with the component production process in progress, forexample), the bonding defects are then prohibitive.

This therefore raises the problem of reducing or even eliminating, inthe case of (direct) molecular bonding, the defects due to degassing atthe bonding interface.

The solutions currently used to overcome the formation of defectsinvolve primarily the removal of water at the bonding interface byusing, in particular, ultra high vacuum (UHV) bonding techniques.However, these techniques are not suitable for industrial use. There arealso techniques that consist of forming channels at the bondinginterface in order to evacuate the by-products of the molecular bondingreaction. Unfortunately, such techniques are destructive and presentusage problems.

A problem is therefore to find a treatment solution for reducing defectsthat enables industrial implementation while using the entire surface.

DESCRIPTION OF THE INVENTION

According to the invention, a process for producing a bond between afirst and a second substrate comprises:

a) a step of preparing surfaces to be assembled,

b) an assembly of these two surfaces, by direct molecular bonding,

c) a heat treatment step involving at least maintaining the temperatureof the surface or of the bonding interface within the range of 50° C. to100° C. for at least one hour.

Step c) also comprises, after the step of maintaining the temperaturewithin the range [50 to 100° C.] for at least one hour, a step ofmaintaining the temperature within the range strictly above 100° C., andbelow 500° C. (i.e. within the range ]100° C. to 500° C.]) for at leastone hour.

The term “substrate” refers to a massive substrate or a substrateconsisting of a stack of a plurality of layers of different types.

This heat treatment according to the invention enables good preparationof the conditions for degassing the surfaces in contact by molecularadhesion.

It makes it possible to minimise the defect density at the bondinginterface. At lower temperatures, such a heat treatment makes itpossible to more easily eliminate the by-products of the degassing ofthe interface, by diffusion at the bonding interface.

The standard heat treatments, at higher temperatures, make it possibleto increase the bonding energies of structures, and/or to create afracture in a zone implanted by one (or more) species, for examplegaseous, prior to the bonding. A treatment according to the inventioncan therefore be a complement to the standard heat treatments, at highertemperatures, which reinforce the bonding.

Treatment steps at one or more temperature(s) above 100° C. can alsohave been performed prior to a treatment according to step c) of theinvention.

According to one embodiment, the invention involves the use ofsuccessive or cumulative heat treatments, by levels, for examplestarting at low temperatures, below 100° C. or 200° C.

A level can comprise a ramp and the actual level temperature, thetemperature at which the system is maintained for a certain period. Allof these parameters (temperature ramp as a function of time,temperatures, duration of levels) may vary in relatively wide ranges.

For example:

-   -   a ramp can be as slow as 0.1° C./min,    -   the successive temperatures can be spaced by 1° C. or by several        ° C. only,    -   the duration of a level can be as short as several tenths of a        second and as long as several hours.

Such a multi-level treatment makes it possible to progressively raisethe temperature from the low range of 50° C. to 100° C., and furtherpromotes the elimination of interface degassing by-products.

The invention also relates to a process for producing a bond between afirst and second substrate, comprising:

-   -   an assembly of these two surfaces, by direct molecular bonding,    -   a heat treatment step, by successive or cumulative levels.

Such a treatment makes it possible to reduce the defect density at thebonding interface.

Again, a level can comprise a ramp and the actual level temperature, thetemperature at which the system is maintained for a certain period. Allof these parameters (temperature ramp as a function of time,temperatures, duration of levels) may vary in relatively wide ranges.

For example:

-   -   a ramp can be as slow as 0.1° C./min,    -   the successive temperatures can be spaced by 1° C. or by several        ° C. only,    -   the duration of a level can be as short as several tenths of a        second and as long as several hours.

Such a multi-level treatment makes it possible to progressively raisethe temperature, for example from a low range such as the range of 50°C. to 100° C., and promotes the elimination of interface degassingby-products.

One of the levels is, for example, around 100° C. for at least 3 or atleast 4 or at least 5 hours.

Regardless of the embodiment of the invention, at least one of thesurfaces to be assembled may have previously been subjected to apreparation step for the purpose of assembly, for example a treatmentstep giving it a hydrophilic or hydrophobic character.

In every case, the assembly can be performed by bonding, under acontrolled atmosphere.

The heat treatments according to the invention, by successive orcumulative levels, can be performed at progressive temperatures.Increases or levels at increasing temperatures can be performed,optionally with returns to a lower temperature, for example roomtemperature, between two levels.

The invention therefore relates in particular to the use of specificheat treatments, short or long, but successive, at low temperatures,preferably below 200° C. or 100° C., as a complement to the standardheat treatments.

For example, specific heat treatments are carried out, with each levellasting around two hours, successively at the following temperaturelevels: 50° C., then 100° C., then 125° C., then 150° C., and finally200° C.

According to another example, heat treatments according to the inventionare carried out cumulatively. For example, a first level is performedfor two hours at 100° C.; then, the temperature is returned to roomtemperature, then maintained at a second level, for two hours, at 150°C. It is then returned to room temperature, then again maintained at athird level, for two hours, but at 200° C. The temperature is thenreturned to room temperature.

These successive heat treatments can in particular be adjusted bymodifying the speeds (or ramps) of increase (or decrease) intemperature, until the desired temperature is reached.

Advantageously, slow increase (or decrease) speeds will be used. Forexample, speeds below 5° C. per minute, or below 1° C. per minute, orbelow 0.1° C. per minute, will be chosen.

These heat treatments, successive or cumulative, can be used incombination with effective surface preparations making it possible toobtain structures free of defects at the bonding interface of thehydrophilic or hydrophobic surfaces. These treatments can be a series ofsurface preparations such as plasmas, or rapid annealings, or bondingenvironments under various atmospheres and various pressures ortemperature bondings.

Heat treatments according to the invention can be followed, for exampleuninterruptedly, by one or more heat treatments, at one or moretemperatures, for example, above the heat treatment temperaturesaccording to the invention, in particular for the purpose of energyreinforcement (bonding).

The invention also relates to a process for producing a thin film on afirst substrate, comprising a process for producing a bond between thefirst substrate and a second substrate as described above, then a stepof thinning the second substrate.

The thinning step can be performed by chemical and/or mechanicalthinning, or by fracture of the second substrate.

In the latter case, the second substrate can be pre-implanted by one (ormore) species, advantageously gaseous, in order to create a zone ofweakness or fracture. This species is preferably implanted at a doseabove the minimum dose enabling the fracture.

For example, the species can be hydrogen.

The implantation can be ionic.

In the case of a crystalline plate, for example made of a semiconductormaterial such as silicon, the implantation can be performed at a doseabove the minimum dose. The fracture can then be induced at atemperature below the temperature normally necessary to cause thefracture at the minimum dose.

If the species implanted is hydrogen, the dose implanted will be, forexample, greater than or equal to 6×10¹⁶H⁺·cm⁻².

Even for “standard” doses (and therefore not only in the case of anoverdose), a heat treatment according to the invention has a benefit, inparticular for the “Smart Cut®” process, by limiting the number ofdefects.

A process according to the invention is particularly suitable for theassembly of two silicon substrates, or two silicon dioxide substrates(or substrates covered with silicon dioxide), or a substrate made of (orcovered by) silicon dioxide and a silicon substrate.

A process according to the invention makes it possible in particular toobtain a thin (thickness below 50 nm) or even ultra-fine oxide film,embedded at the bonding interface.

A process according to the invention also makes it possible to obtainvery thin layers of Si (thickness below 150 nm) or of SiO2 (thicknessbelow 50 nm) directly bonded on Si or SiO₂ plates.

In addition, the bonding surfaces can be diverse, for example chosenfrom semiconductors (Si, SiGe, Ge, III-V, etc.) conductors (Ni, Co, W,Ti, Ta, Pt, Pd, etc.) or insulators (SiO2, Si3N4, AlN, Al2O3, diamond,etc.) alone or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention can be better understood on reading the description ofexamples of embodiments provided purely for indicative and non-limitingpurposes, in reference to the appended drawings, in which:

FIG. 1 shows a pair of substrates to be assembled,

FIGS. 2 to 4 and 12 show various changes in temperature as a function oftime for various heat treatment processes according to this invention,

FIGS. 5A and 5B show two acoustic microscopy images, one (FIG. 5A) aftera standard heat treatment (400° C./2 h), and the other (FIG. 5B) afteran additional slow-ramp heat treatment, and followed by the samestandard heat treatment 400° C./2 h,

FIG. 6 shows the change in the defect density at the interface, due tothe degassing of the species, as a function of the temperature T of abonding consolidation heat treatment, with a heat treatment according tothe invention (squares), and without a heat treatment according to theinvention (circles),

FIG. 7 shows an acoustic microscopy image of the interface of an Si—Sibond after a heat treatment according to the invention and aconsolidation heat treatment at 700° C. for 2 h, without defects,

FIG. 8 is an example of defectiveness observed in interferometry forstructures having thin films treated only by a “standard” process,

FIG. 9 is an example of defectiveness observed in interferometry for astructure having a thin film obtained by a process according to theinvention,

FIG. 10 shows a thin film on a substrate,

FIGS. 11A and 11B show steps of a process for obtaining a structure asshown in FIG. 10.

Identical, similar or equivalent parts of the various figures describedbelow use the same numeric references for the sake of consistencybetween figures.

The various parts shown in the figures are not necessarily shownaccording to a uniform scale, in order to make the figures easier toread.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

An example of an embodiment of the invention will be given in relationto FIG. 1, in which references 2 and 4 designate two substrates to beassembled, with respective assembly surfaces 6 and 8.

These are, for example, silicon plates, plain or covered with finesilicon oxide, with a thickness below 50 nm.

Surfaces 6 and 8 of the pair of substrates 2 and 4 may have beensubjected, prior to the heat treatment according to the invention, to apreparation giving them a hydrophilic or hydrophobic character.

For example, a preparation of a surface with a hydrophilic charactercomprises a chemical treatment of the Sulfo-Peroxide Mixture (SPM)and/or Ammonium Peroxide Mixture (APM) type and/or a treatment enablingfor example a cleaning, such as a (water and/or hydrocarbons) degassingheat treatment, and/or an activation of the surfaces by UV, and/or Ozoneand/or by plasma, for example RIE or microwave, or ICP, etc., undervarious atmospheres.

The bonding can take place under various pressures, with or withoutthermalisation (the latter can be performed, for example, at between200° C. and 300° C.).

According to another example, a preparation of a surface with ahydrophobic character comprises a surface deoxidation treatment; in thecase of a silicon surface, it can be a HF liquid chemical attack.

Substrates 1 and 2 are assembled, one on the other, by the assemblyfaces 6 and 8 prepared before.

To improve the removal of species at the bonding interface, it is alsopossible to perform the bonding under a controlled atmosphere (vacuum orN₂ atmosphere), with or without thermalisation.

Once the bonding has been performed, the structure is subjected,according to the invention, to a heat treatment involving maintainingthe temperature in the range of 50° C. to 100° C. for at least one hour.In this range, the temperature can change or be constant. For example,it can be equal to 100° C. for one hour, or change, starting at 50° C.,according to a thermal ramp of 50° C./h, thus passing 100° C. after onehour (solid line 10 of FIG. 12).

The time passed between 50° C. and 100° C. can also be above 1 hour, or1.5 h, or 2 h, or 2.5 h or 3 h.

The temperature is also maintained for at least one hour, at atemperature strictly above 100° C., and, for example, below 500° C.

An example of a treatment according to the invention involves at leastmaintaining the temperature in the range of 50° C. to 200° C. or between100° C. and 200° C. or between 200° C. and 250° C., for at least onehour or two hours or three hours in order to satisfy the conditions setforth above (temperature between 50° C. and 100° C. for at least onehour and, for at least one hour, temperature strictly above 100° C., forexample below 500° C.).

The system can previously have been subjected to treatments at higher orlower temperatures. Consequently, it is also possible to have apreliminary treatment at over 100° C. or 150° C. or 200° C., then areturn of the temperature to between 50° C. and 100° C., and a treatmentaccording to the invention in particular while maintaining thetemperature in this range of 50° C. to 100° C. for at least one hour ortwo hours or three hours.

Such a treatment according to the invention can be followed by anothertreatment, for example at a higher temperature, in order to reinforcethe bonding or fracturing of one of the substrates as explained below.

The treatment according to the invention led, aside from the maintenanceof the temperature at between 50° C. and 100° C. for at least one hour,to bringing the system to higher temperatures, for example 200° C.,and/or 300° C. and/or other temperatures (this is the case fortreatments with levels as explained below) A treatment according to theinvention can also be followed by a treatment at least at a temperaturebelow the temperature of one of the levels.

A complementary treatment, of the bonding or fracture reinforcementtype, does not necessarily immediately follow a treatment according tothe invention. A step of another intermediate treatment may take placein the meantime.

FIG. 12 shows an example of a treatment comprising:

-   -   a preliminary treatment phase at a temperature T3, for example        to perform the bonding,    -   then a treatment according to the invention (phase I),        comprising:

a) maintaining the system for at least one hour at between 50° C. and100° C. (in fact, the system is maintained in this temperature range fora period longer than one hour, since it is maintained at 100° C. alsoduring phase I′),

b) and, in the particular case represented, a treatment step at atemperature T4, above 100° C.,

-   -   finally, a complementary treatment (phase II), for example to        reinforce the bonding, at a temperature equal to, above (T6) or        below (T5) one of the temperatures of the treatment according to        the invention.

The treatment phase I according to the invention can comprise a ramp,shown with the dotted line in FIG. 12, during which the system slowlygoes from 50° C. to 100° C. over at least one hour. The system is alsomaintained for at least one hour, at a temperature strictly above 100°C., as shown in the zone located in zone I, but beyond I′ (between timest3 and t4), as well as zone II.

An example of a heat treatment according to the invention is in fact atreatment by levels.

Below, we will describe the levels of a heat treatment according to theinvention as:

-   -   successive, when one follows another, without returning to room        temperature or to a lower temperature (for example shown        diagrammatically in FIGS. 2 and 3),    -   cumulative, when one follows another with a return, between two        levels, to a lower temperature, for example room temperature        (for example the treatment shown diagrammatically in FIG. 4).

The heat treatment according to the invention can consist of acombination of successive and/or cumulative levels.

For example, for Si—Si bondings, the low temperature of the levels of aheat treatment according to the invention will be below 200° C. and moreadvantageously below 150° C., for example, equal to or close to 50° C.,then 100° C., then 125° C. or 145° C.

The steps or levels of a heat treatment according to the invention arelong when their durations are more than one hour or two hours or,advantageously, more than five hours.

The duration of a level or a step of a heat treatment according to theinvention includes both the duration of the increase from roomtemperature, the duration of maintaining the temperature of the leveland the duration of decreases from the treatment level temperature to,for example, room temperature.

A heat treatment according to the invention, by levels, can involve, asin the treatments described above, maintaining the temperature withinthe range of 50° C. to 100° C. for at least one hour. In this range, thetemperature can change, or be constant. For example, it can be equal to100° C. for one hour or change, starting at 50° C., according to athermal ramp of 50° C./h, thus passing 100° C. after one hour (dottedline 10 of FIG. 12).

The time passed between 50° C. and 100° C. can also be above 1 hour, or1.5 h, or 2 h, or 2.5 h or 3 h.

Another treatment according to the invention, by levels, involvesmaintaining the temperature within the range of 50° C. to 200° C. orbetween 100° C. and 200° C., or between 200° C. and 250° C., for atleast one hour or two hours or three hours. We will also seek to satisfythe conditions set forth above (temperature for at least one hourbetween 50° C. and 100° C. and, for at least one hour, strictly above100° C., and for example below 500° C.).

According to an example, a treatment of the two substrates 2 and 4 ofFIG. 1 comprises successive temperature levels at low temperatures.

For example, this treatment is performed for a period of around 5 hoursfor each level, and successively at increasing temperatures. A firstlevel can be T1=50° C., a second T2=100° C., a third T3=150° C., and afourth T4=200° C. The change in temperature as a function of time isshown in FIG. 2. It is then possible to progress further by levels of100° C., until reaching a temperature, T, of a bonding reinforcementheat treatment.

According to an alternative, it is possible to implement a very slowtemperature increase ramp making it possible to very gradually raise thetemperature to the levels T1, then T2, then T3, then T4, for examplewith a slope of 1° C./min, or even advantageously 0.1° C./min.Advantageously, the temperature is maintained at successive heat levelsT1, T2, T3 and T4, starting at low temperatures, for example at 100° C.,with each level being maintained, for example, for 10 hours. Such atreatment is shown in FIG. 3.

One or more heat treatments can thus be defined, all enabling a totalheat treatment according to the invention.

A heat treatment according to the invention, by successive or cumulativelevels, can be followed by a heat treatment reinforcing the assembly ofthe two substrates, for example at a temperature above that of the heattreatment levels according to the invention.

Another treatment according to the invention is shown in FIG. 4: levelsare set at temperatures T1, T2, T3 and T4, with returns, between eachlevel, to a lower temperature TO, for example room temperature, forexample at 20° C.

Thus, a cumulative heat treatment can have the following form, startingat room temperature (for example: 20° C.):

-   -   a first level for a period of 2 hours at 50° C., followed by a        return to a lower temperature (for example: room temperature),    -   then a second level for 2 hours at 100° C., followed by a return        to a lower temperature (for example: room temperature),    -   then a third level for 2 hours at 150° C., followed by a return        to a lower temperature (for example: room temperature),    -   then a fourth level for 2 hours at 200° C., followed by a return        to a lower temperature (for example: room temperature),    -   then a standard bonding reinforcement heat treatment at a        temperature T, for example 400° C., for 2 hours.

Another example of a heat treatment according to the invention is a rampbringing the temperature of the system progressively from roomtemperature to a final temperature, which ramp is such that a period ofat least one hour is passed in the range of 50° C. to 100° C. Theduration passed between these two temperatures can also be above 1 h, or1.5 h, or 2 h, or 2.5 h or 3 h. The treatment is then completed by thetreatment steps according to the invention (maintaining the temperaturein the range strictly above 100° C. and below or equal to 500° C. for atleast one hour).

Heat treatments according to the invention have been carried out with awet chemical hydrophilic surface preparation (SPM and APM).

The effects of the various heat treatments according to the inventioncan be compared in terms of defect density.

1) in the first case (table 1), the following were compared:

-   -   a heat treatment according to the invention, of the type of FIG.        3, with a slow ramp of 1° C./min, starting at room temperature,        with levels each lasting 10 h, at 100° C., then at 200° C., then        at 300° C., and finally at 400° C.,    -   and a bonding reinforcement heat treatment, referred to as the        “standard” treatment, which is quasi-isothermal at 400° C.

Table I shows a defect density at the bonding interface that is clearlylower than in the case of the treatment according to the invention.

The images shown in FIGS. 5A and 5B are images of the bonding interfaceobtained by acoustic microscopy. In FIG. 5A, it is an image after the“standard” heat treatment alone. In FIG. 5B, it is an image after theheat treatment according to the invention and after the same standardheat treatment as that of FIG. 5A. FIG. 5B shows, with respect to FIG.5A, an improvement, with the treatment according to the invention, ofthe defectiveness, by a factor greater than 8 (FIGS. 5A and 5B relate tothe results of table I).

The repair of bonding defects, at high temperature (for example above1100° C.) is therefore largely facilitated in a preliminary applicationof a treatment according to the invention.

When such a high-temperature repair treatment is not possible, forexample due to the presence of components in one of the substrates, thetreatment according to the invention makes it possible to considerablylimit the defect density in the final assembly.

TABLE I Defect density at the bonding interface Standard heat treatmentalone 88% (400° C./2 h). Additional heat treatment with 10.60% slow rampbefore the standard treatment at 400° C.

2) in the second case, the following were compared:

-   -   a heat treatment according to the invention, consisting of long        successive levels lasting 5 hours at 50° C./min, followed by 5        hours at 100° C., followed by 5 hours at 150° C., followed by a        heat treatment at temperature T (bonding reinforcement        temperature),    -   and a standard quasi-isothermal heat treatment at temperature        T=200° C. (or 300° C. or 400° C.) for around 2 hours, for        reinforcement of the interface.

An improvement in the defectiveness by at least a factor of 4 is notedowing to the heat treatment according to the invention, and for eachstandard interface reinforcement heat treatment temperature (at 200° C.or 300° C. or 400° C.).

FIG. 6 shows the change in defect density as a function of the annealingtemperature, with a treatment according to the invention (squares) andwithout a treatment according to the invention (circles). The example isthat of an Si—Si bond, with a wet chemical preparation (SPM, APM).

Other application examples can be given.

Example 1

By optimising the preliminary surface preparations, for example bypreparing surfaces 6 and 8 by microwave plasma in an oxygen atmosphereand producing a bond between the two surfaces under vacuum with heatingto 300° C. during the bonding, it was possible to obtain an Si—Si bond,without defects at the bonding interface (as shown in FIG. 7) under thefollowing conditions:

-   -   a heat treatment according to the invention is first performed,        which treatment comprises a slow ramp of 1° C./min, starting at        room temperature, then having levels of a duration of 10 hours        each at 100° C., then at 200° C., then at 300° C., and so on by        levels of 100° C. until reaching the final temperature of 700°        C.,    -   a “standard” bonding reinforcement heat treatment, in the range        of 600 to 700° C.

FIG. 7 shows an acoustic microscopy image of this Si—Si bondinginterface, after heat treatment by levels according to the invention,followed by a consolidation treatment at 700° C. for 2 hours. Thisinterface is free of defects.

Example 2

By chemically preparing the surfaces, for example with an attack by HFin solution, so that they become hydrophobic, it was possible, with aheat treatment according to the invention, to obtain defect-free bondinginterfaces at up to 500° C. and more. The heat treatment according tothe invention is a slow ramp, of 0.15° C./min, starting at roomtemperature, combined with levels, each lasting 10 hours, at 100° C.,then at 200° C., then at 300° C., and so on by levels of 100° C. untilreaching the final temperature of 500° C.

Various other applications of a process according to the invention canbe mentioned.

The use of additional heat treatments according to the invention makesit possible to produce stacked structures by molecular bonding withminimal or even no bonding defects. Among the various applications, itis then possible to produce thin films (for example below 100 μm or 1 μmor 0.1 μm) is possible.

For example, the initial structure is obtained by bonding two thickplates 2 and 4 (FIG. 1), followed by a heat treatment by levelsaccording to the invention, and optionally a reinforcing heat treatment.It is then possible to use mechanical thinning technology (lapping,grinding, etc.) and/or a chemical thinning technique (chemical attack,lift off, etc.) and/or other techniques alone or in combination. Thestructure of FIG. 10 is then obtained with a substrate 2 and a thin film40.

According to another example, at least one of the two thick plates 2, 4has a crystalline surface structure, implanted with a species such as agaseous species, for example by ion implantation, in order to generate aweakness zone 21 (FIG. 11A). Then a heat treatment by levels accordingto the invention is carried out, and optionally a reinforcing heattreatment.

It is then possible to use the technology known as “Smart Cut”(registered trademark): after the bonding of the two thick plates (FIG.11B), a separation is caused, for example in a heat treatment, at thelevel of the weakness zone 21, and the thin film 40, which remainsadhered to the plate 2, is detached (FIG. 10).

The process according to the invention can also advantageously be usedin the following fields of application:

1. Productions of stacked structures by molecular adhesion, includingthin or ultra-thin layers, with a thickness for example below 2 μm oreven 0.1 μm, for example; the production of silicon-on-insulator (SOI)structures with films of silicon and fine buried oxides (BOX). Inparticular, the thickness of the oxide at the bonding interface istypically below 50 nm. Since the oxide does not have the ability toabsorb the degassing products of the bonding, the heat treatmentsaccording to the invention enable these products to disappear withoutdamaging the bonding interface.

2. The production of certain heterostructures, obtained by directbonding, which poorly withstand, or do not withstand, high-temperaturerepair heat treatments, for example:

-   -   two substrates 2, 4 made of materials with excessively different        expansion coefficients, for example with a ratio above 2 and of        which the bonding cannot withstand heat treatments at high        temperatures; this is the case, for example, of silicon on        sapphire, of which the heat expansion coefficients are        respectively 2.5×10⁻⁶ K⁻¹ and 7×10⁻⁶ K⁻¹,    -   two substrates or plates 2, 4 made of materials presenting a        risk of diffusion of an element through the bonding interface;        this is, for example, the case of two plates 2, 4 made of        differently doped semiconductor materials (example: silicon);        according to an example, one is doped with boron and the other        is doped with phosphorus,    -   two substrates or plates 2, 4 to be stacked, capable of being        degraded by a high-temperature heat treatment; for example, one        of the plates is already partially processed, or already has        components, and cannot therefore be exposed to a temperature        above 450° C. (the case of a metal layer of a component on        silicon).

The process according to the invention can also advantageously be usedin the following application.

According to the usual processes described as “standard”, the productionof thin-layer films can be complex when the thickness of the filmsbecomes very low, on the order of several nanometres, or between 1 nmand 10 μm. Indeed, production defects appear on or in the films produced(holes, folds, bubbles/blisters, etc.) in the production of films or inannealings that make it possible to stabilise the new structuresproduced. FIG. 8 is an example of defectiveness observed with a “MagicMirror” apparatus of Hologenix, for structures of thin films treated bythe “standard” process. Several hundred defects make the structureindustrially “unusable”.

This problem concerns in particular the production of SOI(silicon-on-insulator: structure Si/SiO2/Si) materials; it also concernsthe production of SIS (semiconductor-insulator-support) materialscomprising a thin semiconductor layer (from several nanometres, forexample 5 nm, to several μm, for example 5 μm or 10 μm, in thickness),which conducts the electric current according to certain electricalconditions (voltage/current, for example), on an insulator. The lattermakes it possible to insulate the thin layer semiconductor from theunderlying support (SiO2, Si3N4, diamond, etc.). The support makes itpossible to maintain the preceding two thin layers stacked in order tocreate the final industrialised structure.

According to the invention, the defect problems inherent to theso-called “standard” process are solved.

In the case of a heat treatment inducing a fracture, the temperature atwhich the plates are put in the detachment oven is carefully chosen:advantageously, the plates will be introduced at a temperature aboveroom temperature, for example 50° C. or 80° C. or 100° C., or between50° C. and 80° C. or between 80° C. and 100° C., for example at thetemperature of the first temperature level in a heat treatment bylevels, for example at 50° C. or 80° C. or 100° C. The use of arelatively long time, on the order of at least 3 hours or at least 4hours or at least 5 hours, at one or more relatively low temperature(s),for example on the order of 50° C. or 80° C. or 100° C. or 150° C., orfor example between 50° C. and 80° C. or between 80° C. and 100° C. orbetween 100° C. and 150° C. makes it possible to reduce the number ofdefects for plates subjected to plasma activation before bonding.

The substrate fracture can be obtained using the Smart Cut™ or substratefracture technology, described, for example, in “Silicon Wafer BondingTechnology”, edited by S S Iyer and A J Auberton-Hervé, INSPEC,Institute of Electrical Engineers, London, 2002, Chapter 3, p. 35 andfollowing, by B. Aspar and A. J. Auberton-Hervé, in the following way:

It is possible first to implant a dose much higher than that generallyrequired—at least 20% higher (for example a dose of 8×10¹⁶H⁺·cm² for thehydrogen implanted in the silicon oxide, whereas the usual process usesonly 5×10¹⁶H⁺·cm²). It is then possible to use the annealing previouslydescribed, with a placement in a low-temperature oven (below 100° C.).Slow and long temperature increase ramps (0.25° C./min for example)enable the thin film structure of the “Smart Cut” type to be released ata lower temperature than the usual process (for example in the case ofhydrogen in silicon at a temperature below 400° C., for example 300° C.,whereas the fracture normally occurs at 500° C.).

This process (overdose of the species implanted at depth for transfer toa lower temperature for slow and long annealing) makes it possible toproduce a structure several nanometres in thickness with a minimumnumber of defects.

The fracture is therefore obtained at a lower temperature than for thestandard process. It is therefore possible to produce, for example attemperatures below, or on the order of, 400° C., for example 300° C., afracture in structures that are not compatible with the usual fracturetemperatures (around 500° C.). This is the case in particular for aprocessed structure (i.e. comprising, in or on the thin film to betransferred or the receiving substrate, all or some electroniccomponents (CMOS, for example) or others (MEMS, MOEMS, etc.) orcomprising metal interconnections, etc.).

Thus, in FIG. 9, it is noted that a thin layer obtained by a processaccording to the invention contains fewer than 10 defects, whereas morethan one thousand defects are observed in the “standard” process (FIG.8).

Consequently, according to an embodiment of the invention, favourableion implantation conditions are selected: overdosed implanted species,for example at a dose above the minimum dose enabling a fracture (above6×10¹⁶ H⁺·cm² or 7×10¹⁶ H⁺·cm², for example, for hydrogen). Theseconditions make it possible, at low temperature, to produce structureshaving very low film thicknesses (of several nanometres) with aconsiderably reduced defect density, from several hundred or severalthousand to just several units, or even without defects.

It is thus possible to produce thin film structures (semiconductor, forexample) on a thin film (insulator or not), all on a support.

An example of the use of an annealing process is as follows. Asexplained above, it is sought to overdose an ion or atom implantation inorder to produce a fracture at a lower temperature than in the knownprocesses.

According to this example, a silicon oxide plate is implanted with H⁺ions at a dose of 8×10¹⁶ H⁺·cm⁻² and an energy of 50 keV.

It is bonded to another Si plate, by an oxide layer, and a Si/SiO₂/Sistructure is thus obtained, for example with an oxide thickness of 12nm.

Then, the following annealing cycle is carried out:

-   -   the temperature is initially 100° C., then the isotherm 100° C.        is maintained for 10 hours,    -   a ramp at 0.25° C./min is then produced, until reaching the        isotherm 200° C., maintained for 10 hours,    -   a ramp at 0.25° C./min is then produced, until reaching the        isotherm 300° C., maintained for 10 hours,    -   again a ramp at 0.25° C./min is produced until reaching the        isotherm at 400° C., maintained for 10 hours; the fracturing of        the substrate is produced during this step,    -   again a ramp at 0.25° C./min is produced, and an output        temperature of 200° C. is reached.

According to an alternative, an implantation is performed with a dose of8×10¹⁶ H⁺·cm² at 76 keV, which will enable the transfer of 700nanometres of Si.

The annealing cycle is as follows:

-   -   the temperature is initially at 100° C., then the isotherm is        maintained at 100° C. for 10 hours.    -   a ramp at 0.25° C./min is then produced, until reaching the        isotherm 200° C., maintained for 10 hours,    -   a ramp at 0.25° C./min is then produced, until reaching the        isotherm 300° C., maintained for 15 hours; the fracturing of the        substrate is performed before this step,    -   again a ramp at 0.25° C./min is produced, and an output        temperature of 200° C. is reached.

According to yet another example, an implantation is produced at a doseof 6×10¹⁶ H⁺ ions at 210 keV through a thermal oxide, which will enablethe transfer of 1.56 μm of Si.

Two plates or substrates 2 and 4 of silicon are selected, of which oneis oxidised at the surface, for example over a thickness of 0.4 μm. Thisoxidised plate is then implanted with H⁺ ions, with the dose and energyindicated above, then it is deoxidised. The implanted zone forms afracture zone such as zone 21 of FIG. 11A, which will subsequentlyenable a thin layer to be separated from the remainder of the substrate.

The two plates are then cleaned by RCA chemistry and their surface isactivated by plasma.

The plates are then placed under vacuum (10⁻³ mbar) with a temperatureincrease to 300° C. (heat ramp of 20° C./min). They are maintained atthis temperature for 10 minutes.

The bonding is then induced at this temperature for a period of twohours, then the system is returned to room temperature.

A treatment according to the invention is then applied, with atemperature ramp of 1° C./min, starting at room temperature, up to 100°C. The following is then performed:

-   -   the temperature is maintained at 100° C. for 10 hours,    -   then a level at 200° C. for 10 hours,    -   then a level at 300° C. for 10 hours,    -   then a level at 400° C. for 10 hours.

The fracture treatment is then induced during the final level at 400°C., resulting in a transfer of a silicon film of 1.56 μm.

In this example, the system is subjected, before the heat treatmentaccording to the invention, to a treatment at a temperature above 100°C.

The invention also relates to the case of “standard” implantation doses(and therefore not only overdose cases as in the examples alreadyprovided); a heat treatment according to the invention then has aninterest in the implementation of the “Smart Cut®” process, by limitingthe number of defects. An example will be provided, which shows thedetachment annealing of bonded plates, for a transfer according to the“Smart Cut®” process:

-   -   the silicon donor plate has an oxide layer on the order of 50 nm        of thickness,    -   it is activated by an O2 plasma treatment at 535 W for 45 s,    -   it is implanted with hydrogen at a dose on the order of 10¹⁶ H⁺        ions/cm² and an energy on the order of 30 keV,    -   the detachment annealing is performed by exposing the plates to        a temperature of around 100° C., for at least 5 hours, then an        increase in temperature by 0.5° C./min to 200° C., then        maintaining the temperature at 200° C. for 2 hours, and,        finally, increasing the temperature by 0.5° C./min to 500° C.

The transfer of the layer to the receiving plate is thus performed withfewer than 5 pinhole-type defects.

In all of the experiments and examples described, no additionalmechanical force is applied to create the fracture of the implantedsubstrate.

1-30. (canceled)
 31. Process for producing a bond between a first and asecond substrate, comprising: a) a step of preparing surfaces to beassembled, b) an assembly of these two surfaces, by direct molecularbonding, c) a heat treatment step involving at least maintaining thetemperature within the range of 50° C. to 100° C. for at least one hour,then maintaining the temperature in the range strictly above 100° C. andbelow or equal to 500° C. for at least one hour.
 32. Process accordingto claim 31, said step c) comprising a passage through successive and/orcumulative temperature levels.
 33. Process according to claim 32, saidtemperature levels being successive, without a return to roomtemperature.
 34. Process according to claim 32, said temperature levelsbeing cumulative, with, between two successive temperature levels, areturn to a temperature below the temperatures of two levels. 35.Process according to claim 34, the lower temperatures between twosuccessive levels being all identical.
 36. Process according to claim35, the lower temperatures between two successive levels being all equalto room temperature.
 37. Process according to claim 32, the temperaturelevels being produced at temperatures increasing over time.
 38. Processaccording to claim 32, at least one of the temperature levels comprisinga rate of temperature increase below 5° C. per minute.
 39. Processaccording to claim 31, said step of preparing the surfaces being ahydrophilic or hydrophobic treatment step.
 40. Process according toclaim 31, said assembly being produced by bonding, under a controlledatmosphere.
 41. Process according to claim 31, at least one of the twosubstrates being a semiconductor material.
 42. Process according toclaim 31, at least one of the two substrates being made of silicon. 43.Process according to claim 31, the two substrates being made of silicon.44. Process according to claim 31, the two substrates at least having asilicon dioxide surface.
 45. Process according to claim 31, one of thetwo substrates having at least a surface of silicon dioxide and theother is silicon.
 46. Process according to claim 31, the two substratesbeing made of materials with different heat expansion coefficients. 47.Process according to claim 31, at least one of the two substratescomprising at least one component.
 48. Process according to claim 31,the bonding being performed under a controlled atmosphere, undercontrolled pressure, with or without thermalisation.
 49. Processaccording to claim 31, the temperature at the end of step c) being aheat treatment temperature for reinforcing molecular bonding and/orinducing a fracture in a substrate.
 50. Process according to claim 31,also comprising a step: d) of reinforcing the bonding by molecularadhesion and/or inducing a fracture in a substrate.
 51. Processaccording to claim 50, step d) being performed by a heat treatment at atemperature above the temperatures of step c).
 52. Process according toclaim 50, wherein, during step c), the system is brought, for example byone or more temperature levels, to a temperature above 100° C., withstep d) being performed at least at a temperature above, or equal, orbelow this temperature above 100° C.
 53. Process for producing a thinfilm on a first substrate, comprising a process for producing a bondbetween the first substrate and a second substrate according to claim31, then a step of thinning the second substrate.
 54. Process accordingto claim 53, the thinning step being performed by chemical and/ormechanical thinning.
 55. Process according to claim 54, the thinningstep being performed by fracturing the second substrate.
 56. Processaccording to claim 54, the second substrate being pre-implanted by oneor more atomic or ionic species in order to create in it a weaknesszone.
 57. Process according to claim 56, the atomic or ionic speciesbeing implanted at a dose above the minimum dose enabling the fracture,which is performed at a temperature below or equal to the temperaturenormally associated with the minimum dose.
 58. Process according toclaim 57, the fracture being performed at one or more temperature(s)between 50° C. and 150° C., for at least 3 hours.
 59. Process accordingto claim 57, the ionic species, H⁺, being implanted in silicon at a doseabove 6×10¹⁶ H⁺·cm⁻².
 60. Process according to claim 53, the thin filmobtained having a thickness below 1 μm or 100 nm or 50 nm.